Cables for power, electronic (telephone) transmission, hydrophone cables for oil exploration at sea and other various uses have been filled with various substances in order to protect against water intrusion since 1970. Intrusion occurs when water penetrates into a localized opening in a cable sheath and is free to channel as far as physical processes for water spread and transport allow, often hundreds of feet. Not only does this upset capacitance balance of the transmission cable line but it introduces more potential corrosion sites in proproportion to the length of wire that is wetted. The useful life of water-soaked wires is obviously shorter than that of dry wires.
The solution that has been widely adopted is to fill the voids in the cable with a water insoluble filling material that simply encapsulates the cable components to prevent water intrusion. The filling fluid serves four purposes: (1) it provides a sonic couple of the hydrophone with the jacket; (2) it protects the electrical wiring from corrosive salt water; (3) it protects the environment from oil spills, and (4) it protects a vessel crew from the dangers associated with handling harsh solvents and oils. However, although this physical function of the cable filling material is straight-forward, the choice of the material is not. Among the many considerations that are important for materials used in this application are the hydrophobic nature of the material, low temperature properties, flow characteristics at elevated temperatures, the highest temperature at which the encapsulant may be used ("upper service temperature"), processing characteristics, handling characteristics, dielectric properties, toxicity and cost. In addition, it is desired that the material have the capacity to control the gellation or gel dissolution temperature by changing the character of the gel. This is so that a specific gel dissolution temperature may be selected.
Materials that satisfy most of these criteria and which have been widely used are described in U.S. Pat. Nos. 3,607,487, 3,717,716 issued Sept. 21, 1971 and Feb. 20, 1973 respectively and U.S. Pat. No. 3,775,548. These materials are essentially a petroleum jelly, mixed with a polymer, usually polyethylene, to impart consistency and prevent flowing at warm temperatures below the upper service temperature.
Similar hydrophobic encapsulants have been proposed for filling splice closures. For example, U.S. Pat. No. 3,879,575 issued Apr. 22, 1975 describes a mixture of a low viscosity oil gelled by a styrene-isoprene-styrene copolymer, again with polyethylene added. The polyethylene is used to obtain a high use temperature needed for use with power transmission cables which reach high temperatures. U.S. Pat. No. 4,259,540 discloses the use of a styrene-ethylene butylene-styrene block copolymer, polyethylene, and a paraffinic or napthenic oil, where the oil has a maximum of 5% aromatic oils, in order to enable the cable encapsulant to meet the functional requirements of the cable, i.e., high temperature resistance and to provide good handling characteristics that petroleum jelly material does not possess.
However, all of these above-described encapsulants have a predetermined gel dissolution temperature which cannot be selectively controlled. Control of the gellation (i.e., dissolution) temperature, as well as maintenance of the other necessary and desirable factors discussed, is thus highly desirable; however; heretofore there has not been found an encapsulating material with the necessary advantages, which is also highly selective with regard to the gellation temperature desired. For example, one might wish a gel dissolution temperature to be fairly high so that the gel would form fast upon filling and allow the cable to be used quickly. Or, one might desire a gel which forms at a low temperature so that it would be slower forming. In addition, in none of these previously used materials in the above-described patents may the density be selectively controlled for the amount of buoyancy desired, while still retaining gellation temperature control, etc.
In addition, an encapsulant which is thermally reversible has long been sought. This means that the encapsulant may be removed and replaced during maintenance time and time again at a temperature below the temperature that would damage the cables. An encapsulant which is thermally reversible can be heated to a liquid and then cooled to a gel over and over again without damage to the nature of the filling material, or cable components. This is especially true in hydrophone cables that are generally not permanently installed but towed at sea where the utility of such invention is paramount.